The present invention relates generally to programmable implantable pacemakers, and particularly to implantable dual-chamber pacemakers used for performing noninvasive electrophysiological ("EP") testing. More particularly, the present invention relates to a dual-chamber pacemaker in which the atrial and ventricular channels can operate simultaneously yet independently so that, during noninvasive EP testing for atrial tachycardias, the device can generate bursts of pacing pulses to induce and terminate atrial arrhythmias, while maintaining ventricular pacing support.
EP testing is a procedure that is commonly used to evaluate an individual's susceptibility to cardiac arrhythmias, particularly atrial and ventricular tachycardias. EP testing is also used to determine whether or not a particular patient would respond favorably to various therapies, such as drug therapies and electrical stimulation therapies, that are often used to treat cardiac arrhythmias.
These objectives are typically accomplished by inducing arrhythmias at selected locations in the patient's heart through the use of "bursts" of electrical stimulation. The stimulation bursts are applied to the patient's heart in a sequence that is known to induce the desired arrhythmia.
Once an arrhythmia has been induced, electrical stimulation and/or drug therapy may be used to attempt to revert the arrhythmia. For example, another burst of electrical stimulation may be applied to the patient's heart in a sequence that is known to be successful in reverting arrhythmias. Alternatively, a drug may be administered to attempt to return the patient's heart rhythm back to normal. In addition, EP testing can be used to evaluate the effectiveness of preventative measures, such as drugs that are intended to reduce susceptibility to cardiac arrhythmias in patients who may be predisposed to such episodes. Thus, EP testing allows a physician to prescribe a course of therapy that is specifically tailored to each patient's condition.
EP testing traditionally has been an invasive procedure. Specifically, surgery has been required to introduce electrical leads into the patient's body and to guide the electrode tips of the leads to a desired location in the patient's heart. The leads are coupled to an external EP stimulator which is used by the physician to control the intensity and sequence of electrical stimulation that is delivered to the patient's heart.
Surgery, of course, is not without risks. In the case of invasive EP testing, certain patients may occasionally experience thrombosis, bleeding or infection. Not surprisingly, alternatives to invasive EP testing were sought.
It has been found that noninvasive EP testing can be performed on patients who have received pacemakers to treat bradycardia (slow heart rate). Essentially, the implanted pacemaker, when properly configured, can be used as an in vivo EP testing laboratory. This is typically accomplished by configuring the pacemaker, using an external programming unit, to generate and administer bursts of pacing pulses in a sequence that induces or reverts the desired arrhythmia. Noninvasive EP testing can be used for most tests that are performed through invasive EP testing.
A detailed description of some of the different approaches to noninvasive EP testing, and their respective advantages and disadvantages, may be found in Fletcher, R. D. et al., "The Use of the Implanted Pacemaker as an In Vivo Electrophysiology Laboratory," Journal of Electrophysiology, Vol. 1, No. 5, 1987. In one approach known as "triggered," an EP stimulator is used to apply external, chest wall stimulation to a patient undergoing EP testing. With this approach, the implanted pacemaker is set to a triggered mode of operation (e.g., AAT for atrial testing or VVT for ventricular testing). The chest wall stimulation is applied in a burst sequence designed to either induce or revert an arrhythmia, but at an energy level that is not uncomfortable for the patient. Each stimulation pulse in the burst triggers the implanted pacemaker to administer a pacing pulse. The pacemaker thus tracks the burst sequence of the EP stimulator.
In a second approach known as "indirect," an EP stimulator is coupled to a pacemaker programmer, which in turn is set up to communicate telemetrically with the implanted pacemaker. The programmer maintains a radio frequency link to the pacemaker during the test, except when the EP stimulator sends a pulse to the programmer. When a pulse is sent, the programmer breaks the radio frequency link which causes the pacemaker (which is typically set to the AAI or VVI mode) to administer a pacing pulse. The EP stimulator can thus cause the pacemaker to deliver pulses in a desired burst sequence by sending signals to the programmer in the appropriate sequence.
A third approach known as "direct" does not require an EP stimulator. With this approach, the physician can control the pacemaker burst sequences by using a pacemaker programmer that includes software that is specifically designed for EP testing. When this approach is used, the physician typically first disables pacing support in the chamber that is not to be tested. The physician then uses the programmer to send commands to the pacemaker to cause the pacemaker to administer burst stimulation in a desired sequence to either induce or revert an arrhythmia.
Regardless of the approach used, noninvasive EP testing can be a highly desirable alternative to invasive EP testing. Unfortunately, however, atrial EP testing does not accommodate patients who may require back-up ventricular pacing during the EP test, even if the patient is equipped with a dual-chamber pacemaker. This is because the pacemaker is generally set to a single-chamber mode of operation prior to EP testing. This may be done by setting the pacemaker to AAI or AAT mode (depending on the approach being used to perform EP testing), or by setting the pacing parameters in the non-tested chamber (in this case, the ventricle) to prevent effective pacing pulses from being delivered (e.g., by extending the refractory period or by setting the output energy to below threshold). Dual-chamber pacemakers have generally been set to a single-chamber mode during EP testing in order to prevent high-rate pacing in one chamber (e.g., the atrium) during EP testing from causing the other chamber (e.g., the ventricle) to follow the high-rate activity, because inappropriate pacing of the other chamber may cause pacemaker-induced arrhythmia. Thus, with current dual-chamber pacemakers, if the pacemaker is set to perform EP testing in the atrium, then no ventricular pacing (or ventricular sensing) is provided.
Patients who lack sufficient AV conduction require back-up ventricular pacing during atrial EP testing. Previously known techniques for providing back-up ventricular pacing during atrial EP testing require the insertion of invasive leads into the patient to pace the ventricle.
Thus, it would be desirable for an implanted pacemaker capable of operating in an EP testing mode to provide back-up ventricular pacing support during atrial EP testing. It would also be desirable for the pacemaker to allow operation of the atrial channel involved in EP testing to be decoupled from the operation of the ventricular channel involved in ventricular pacing. It would further be desirable for the pacemaker to allow the physician to set pacemaker parameters for the atrial channel independent of the pacemaker parameters for the ventricular channel, so that the pacemaker can simultaneously perform atrial EP testing and provide optimal ventricular pacing support.